Surprising predictions part 4

 Make Your Own Hygrometer: Sling Psychrometer

A sling psychrometer allows you to quickly and accurately measure the relative humidity in your location with only basic materials and easily performed actions. In order to make your own sling psychrometer, you’ll need a clean glass jar with an airtight lid, two strips of thin cloth, and some water. You can complete this project in 15 minutes or less, so gather up your materials and read on to learn how to make your own sling psychrometer.

How To Use a Sling Psychrometer

A sling psychrometer is an easy-to-use tool that measures relative humidity. When the device is swung in a forward direction, it will reach a higher temperature and when the device is swung in a backward direction, it will reach a lower temperature. The relative humidity can be calculated using this data by dividing the difference between the swings by ten and then multiplying by 100.

1) Find two wooden boards to use as your swing's frame-- one board should be wider than the other for stability. Make sure there is about 2 feet of space between them on either side so you can easily swing your arm through when collecting data from both directions. Attach each board with three screws and use wood glue to hold together if necessary. 

2) Locate a string or cord that is at least 12 inches long (you'll want plenty of extra length). 

3) Tie one end of the string securely to the smaller plank (this should always be located on the left side), leaving plenty of slack on the right end. Tie the other end securely to a nail or screw located near the top right corner of your larger plank (this must always be located on the right side). Make sure it isn't too tight; if you have some slack in this knot, go ahead and tighten it up until you're satisfied with how loose it feels. Now tie another knot around the nail or screw at the opposite end of your plank. Pull on this new knot to make sure it has enough slack and make any adjustments needed before tightening again. That's all there is to setting up your psychrometer!

The process for swinging the device in order to measure relative humidity is simple: Hold onto the sides of both planks tightly, take a step back with one foot and pull back firmly before pushing forward quickly. Repeat these steps until you've collected enough data from each direction. If you need to stop mid-swing, remember to put the boards down carefully on their frames-- otherwise they might break! Once you've finished collecting data, open Excel and plug in all of your numbers into the following equation: (((A1-B1)/10)*100)+((A2-B2)/10)*100. Voila! You now know the relative humidity in your room, which will help you adjust your AC accordingly to prevent mold growth and other damage to furniture. To learn more about different ways to calculate relative humidity, check out our blog post entitled How To Calculate Relative Humidity. We also recommend checking out this video from NC State University called How Do I Determine Air Moisture Content? It walks you through the details of calculating air moisture content, and even includes a helpful PowerPoint presentation to download.

Where Can I Get A Supplies For This?

In order to make a homemade hygrometer, you'll need the following supplies: 

a 2-liter soda bottle, freezer paper or graph paper, a marker and scissors. Simply cut the freezer paper into a rectangle of any size up to approximately 12 x 18. Draw on it with the marker about five 1 boxes for water droplets. Remember to write box 1 at the top left-hand corner of your rectangle. Cut them out and then stick them on your soda bottle so that each box is directly over one of the bubbles. Next, using your friend's skin temperature or ambient air temperature as a reference point (where necessary), place water droplets in three consecutive boxes along your temperature gradient line from high to low. For example, if the skin temperature reading was 37 degrees Fahrenheit, put one drop of water in box 6 and two drops in box 5. If the ambient air temperature reading was 68 degrees Fahrenheit, put one drop of water in box 4 and two drops in boxes 3. Once you have done this take measurements every 15 minutes for an hour. 

The average humidity will be shown by looking at which column has more droplets--the higher column is showing higher humidity levels. The temperatures are also depicted here, even though they are not really needed. The picture below shows what it would look like after the time period has elapsed. Notice how some of the water droplets have evaporated and others did not? That's because these two bottles had different surface areas exposed to air currents. One was covered completely, while the other only covered 60% of its surface area. Also notice how some spots have more than one droplet whereas others don't? 

This can happen when there is condensation around the outside of the bottle. With an open surface area and a limited number of droplets, the specific humidity varies greatly. Compare this to box 1 where all six cells contain at least one droplet--that's because there isn't much open space available for condensation! Notice how we can also see the relative surface areas of both containers on the right side--box 1 covers 100% of its surface area, while box 2 only covers 60%. Boxes 3 through 6 show similar patterns. All but one cell of box 6 still contains a single droplet, but now the other five cells have either two or three droplets. Likewise, four out of six cells in box 5 contain more than one droplet. And finally, four out of six cells in box 4 have either two or three droplets too. At this point most of the water on the bottom row has evaporated and dropped down to fill lower rows with new data points--this is why I suggest doing this test every fifteen minutes instead of hourly; otherwise there might not be enough data points to draw conclusions from! After waiting 45 minutes, the second row begins to dry out too and new droplets start to form on the third row. Notice how none of the droplets were present in box 1 until the 45 minute mark? This means that during this first hour, there were never any moments where conditions were identical between these two containers. After finishing our experiment, it should be clear to see which container had a higher relative humidity throughout this first hour! This is due to the difference in surface area--a smaller surface area leads to greater humidity. This is something that you must keep in mind when choosing a container to use as a humidor. It is imperative that the entire surface of your container have adequate ventilation and air circulation, otherwise mold could grow on your cigars. 

In addition, the graph below shows the relative humidity for box 1 and box 2 throughout this first hour. There were two instances where both containers experienced the same condition, but one container always held higher humidity levels than the other. The times marked A and B correspond to times when both surfaces had a small amount of condensation on their surfaces. As soon as the droplets evaporate, the humidity levels go back up to their original values. Additionally, C corresponds to when both surfaces went without any condensation for 10 minutes before having another bout of condensation (the duration is indicated by the arrow). Finally, D marks another 10 minute window without any condensed water before experiencing yet another round of condensed water (with no interruption). From this graph we can clearly see that there were many more intervals where Box 1 maintained a higher relative humidity level than Box 2 over the course of this experiment. The worst time for box 1 was the moment when both boxes were dry--at this point, Box 1 rose to 75% relative humidity and Box 2 remained at 50%. This is likely because there is a lot more surface area on the top of box 1 than on the top of box 2. The conclusion of this experiment is that a humidor needs to have plenty of open space so that moisture can escape--otherwise, mold will grow on your cigars. You'll want to avoid using a jar in place of a proper humidor. Jars often have narrow openings and sealed lids, making them far less suitable than an actual humidor.

What If I Want An Already Made One?

If you don't have time to learn how to make your own hygrometer, that's okay! There are many different types of hygrometers available for purchase. You can buy a psychrometric hygrometer which is used in thermodynamics and engineering. This will give you a reading from 10% humidity up to 100% humidity. For greater accuracy at lower ranges, we recommend buying a sling psychometer, also known as a sling psychrometer. It has a metal wire inside with two cloth straps on either side. When it's humid outside, the wet side expands more than the dry side and the wire becomes tauter. When it's dry outside, the opposite happens. To use it, find a tree branch or something else sturdy to attach it to and drape one strap over each arm so they hang down freely without touching anything else (they need air flow). Now wait about five minutes for them to reach equilibrium - meaning that one doesn't change size any more than the other does - then read where they meet on the wire by looking through both cloths at once (if you're not sure what this means or can't do it yourself just go here). Now pull the straps tight again and look where they meet. The number you see next to the dot indicates how much humidity there is out there right now. A number between 0-10 means very low humidity, 10-20 indicates low, 20-30 indicates medium range, 30-40 indicates high range, 40-50 indicates very high range and 50+ indicates extremely high range. It may take some practice before you get the hang of this device, but with some patience you'll be able to make an accurate measurement every time. Once you know what the numbers mean, it should be easy to figure out if you need to bring an umbrella tomorrow morning. But since we know that predicting the weather is tricky, why not keep all of your options open? Make a few different kinds of homemade hygrometers and see which one gives you the most reliable readings. That way, even if you have to walk around in heavy rain because you forgot your umbrella and end up soaked head to toe, at least you won't have been caught off guard by being unprepared for snowflakes later in the day! You can also make a moss stick, a mercury glass ball and an improvised sling psychometer. All three of these methods require taking measurements from the surface of the water droplets on leaves or flowers; moss sticks involve hanging a straw vertically near plants and measuring how far water climbs up it, mercury glass balls involve dipping it into liquid like rainwater collected in jars to measure its volume after standing for 5 minutes (this method is less accurate), while slings require hanging objects like copper tubing upside down nearby plants and measuring their distance away after five minutes. All three of these methods work best when done together since they give different information about conditions. Moss sticks give you a sense of humidity changes throughout the day, the mercury glass ball measures how much water is in the atmosphere and an improvised sling psychometer can tell you how quickly it's changing. The only problem is that you might have to think ahead and make a lot of these devices in advance, because they can't be made quickly or easily. 

It's also important to remember that your homemade hygrometer is only a good indicator of the humidity in the immediate area it's located in. You might have different readings for different areas of your yard, for example. And remember that this type of device isn't any more accurate than what you're already using- unless you really want to put some time and effort into making something neat!